Ground-fault protectors for alternating current distribution circuits have attracted increasing interest and concern recently, especially of the type having high sensitivity for protecting people against dangerous shocks due to line-to-ground leakage. The National Electric Code now provides for certain applications of ground-fault circuit interrupters activated in response to 5 milliamperes of line-to-ground leakage. The most practical type of ground leakage detector includes a differential current transformer (DCT) having a common core that encircles both (or all) of the conductors to the load. A secondary winding on the core normally provides no output, assuming the load currents in the conductors produce mutually cancelling magentic fields in the core. Any line-to-ground leakage represents a current in one line conductor that returns to ground by a path other than the neutral conductor. Such a current produces uncancelled flux in the core, resulting in output from the DCT to trip the circuit interrupter.
A low-resistance path from the neutral to ground might be present when a hazardous line-to-ground leakage path occurs. A neutral-to-ground fault tends to reduce the sensitivity of a DCT as a ground fault sensing device, because a neutral-to-ground fault tends to provide a return current path via the neutral for a large percentage of the line-to-ground leakage current. To the extent that line-to-ground leakage current returns to the source via the neutral, that escapes detection by the DCT. Consequently the DCT loses sensitivity and may not respond to an actual 5 milliampere ground leakage current (a hazardous level in the NEC code) where there is a low-resistance neutral-to-ground fault.
Various devices have heretofore been proposed for taking into account the possibility of a ground-to-neutral fault which could reduce the sensitivity of a ground fault detector, for example U.S. Pat. No. 3,473,091 issued Oct. 18, 1969 to Morris et al. Such devices have various disadvantages, in terms of cost and complexity, and in terms of bulk of the apparatus that becomes important in applications where compactness is required.